On Sat, 23 Jun 2001, Dan Michaels wrote:

> Dale.B wrote:
> .....
>  The LEDs are driven via row/column
> >transistors with no current limiting resistors (reduced parts count)
> ......
>
> Dale, do you really think this is safe?

Yep, sure do.  Otherwise I wouldn't have designed it that way.  There
actually is some current limiting provided by the transistors, of course.
Since the LEDs are only on for a few milliseconds at most, The average
current is still well within a reasonable range.  The LED array scanning
is interrupt driven using TMR0, and the watchdog is set to prevent any
unfortunate results should the software glitch.  The maximum duty cycle of
the LEDs in my case is 1/33 (or roughly 3%).  So, yes, I think it's safe.

It was difficult to come up with another way to drive a large-ish array of
LEDs with acceptable brightness, and without a lot of power dissipation
somewhere...  the variable brightness was just a bonus.

Dale
--
A train stops at a train station.  A bus stops at a bus station.
On my desk I have a workstation...

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